The lungs of euthyroid and hyperthyroid rats and rabbits will be investigated by light and electron microscopic methods in order to recognize possible changes in the surfactant, the cells producing the surfactant, and collagenous and elastic elements. The mechanical properties of the lungs and the surfactant from the control and experimental animals will be studied also. Clinical observations and preliminary work on hyperthyroid animals indicate that the proposed experiments and observations hold promise of revealing the pathomorphology and physiology responsible for the reduced lung compliance in hyperthyroid humans.